You know, of the programming error variety?
My favorite bug (and the only cool one I can think of ATM) is in Baldur’s Gate II. To trigger it, you have to be using a magic user that has the follwing spells: simulacrum and project image. Cast project image first, then have the resulting copy of you cast simulacrum. Control the simulacrum and he’ll say everything in the voice of Jon Irenicus. (Why they even recorded the Irenicus voice actor saying command acknowledgements is beyond me.)
I’ve never seen that game before, but as a programmer I can tell you with confidence that that ain’t a bug.
You think? I figured out why they probably had the voice clips recorded (it is a game that allows for mind control, after all), but having passed the game and (later) going through a walkthrough, I can’t find any point in the game where there would be a voice change for the main character. Much less a simulacrum of a projected image of one. Maybe the message is “there’s a little evil voice in all of us”? 
::smacks forehead::
You meant that they included it as an easter egg, didn’t you?
sigh
Don’t mind me, it was a really long day at work.
Sometimes in Super Mario Bros 2, there would be two Mousers, one four inches in front of the other, and the second one would throw a bomb by the first one, and hurt him! (and since they counted as the same Mouser, himself as well!!) I have no clue how it is caused, but saw other people report it, so it isn’t that uncommon.
Do computer games count? Because in Scorched Earth, the little tank that looks like an artillery piece can fly and drive straight through mountains. Tremendous advantages, if you know how to use it right. I think it’s due to the game using video memory to actually keep track of things, so if you can make something look like it’ll work, it actually will. There’s also a way for the human player to get a Triple-Turret Tank, like the computers use, but it’s not actually all that useful.
In the legend of zelda part 2 for the NES, I recall one time attaining level 0 for my character through some bug. Being at level 0 made me extremely powerful and nearly unstoppable.
I know there are plenty more that I’ve seen. I’l lpost them if I remember.
Cool!!!
A scorched earth glitch? Tell me more!
-Soup
In Mario 3, there is a very labrynthine (spelling?) level that involves going through many doors in the right order to get to the end.
I could never figure out how to actually do this, but I did find a way to get there by going through a door that is right next to a wall. On the other side of this wall is the end boss of the level.
If you position yourself so that you are only half on the door and go through it, when you come out on the other side you are partway through the wall. Mario then proceeds to slide through the wall to the right (like what happens when you squat somewhere small and stand up. You just move through the bricks), and you can get to the boss.
In Baldur’s Gate II, there was a period of time where if I selected something in my inventory (so that it was suspended in mid-air) and the main character did a quick save at the same time, then I could duplicate the item. It came in very handy for creating an endless supply of arrows. Unfortunately, my boyfriend wised up to my tactics and downloaded a patch to fix the bug.
And in Act III of Diablo II, I did some combination of key pressing/mouse clicking that made my character walk backwards. He would still fight normally, but as soon as I made him walk, he’d turn around again. He looked a little like he was moonwalking.
I think everyone remembers this one from Super Mario on the First NES.
In the first stage, getting near the end, there was a “block staircase” with two of those weird turkey-turtle looking mutation characters (Which I call Turkeydles) coming down.
Apparantly, jumping just so will make it so that you can bounce almost endlessly on top of it to rack up mega score and dozens of free lives. Eventually, your lives will exceed the limit of the counter, and you’ll begin showing weird characters and graphics instead of numbers. If you die once after that point, it’s game over.
I can’t believe I’m the first to mention the infamous “lumber bug” in Warcraft II. Here’s the deal:
You have to be playing a multiplayer game. I don’t know if it’s necessary to start with “one peasant only”, but the bug will make the greatest difference in that sort of game. Before you build your town hall, tell your peasant to Harvest the nearest stand of trees, but DON’T let him actually complete the Harvest. After telling him to Harvest, redirect him to building the Town Hall. When the Town Hall is complete, you’ll get an extra 100 lumber units as though you had actually chopped down the trees. This can give you an incredible edge in low resource, one peasant only games, but pretty much everyone knows about the lumber bug by now, so it’s hardly “cheating”.
At least three separate glitches, actually:
1: To tunnel through a mountain, drive up next to it, as close as you can get without starting to climb. Then, turn your turret the other direction, and move in the original direction two more spaces. Turn turret, move two, repeat. Note: If you do this with your turret straight up, to create a tunnel of maximum height, then 1000 power, angle 4 will exactly graze the roof of your tunnel, for the best range you can get in the tunnel.
2: To levitate: Just move left-right-left-right, without leaving “fuel” mode. Each step will also take you one pixel up. I often use this to get above other tanks and “body-slam” them: If they don’t have shields, it costs them 50% life, but only costs you 10%. I also use it in conjunction with tunneling (see above) to create sinkholes under enemy tanks for them to fall into. On a cave level, if you can get to the top dirt, you can drive around or tunnel normally in it, but this doesn’t seem to be incredibly useful. Note also: This uses up a lot of fuel, so buy 99 fuel if you want to use it.
3: To “fly” horizontally: Position the mouse cursor a little off the edge of the cliff, and drive onto it. Move the cursor over a little more, and keep driving. Don’t exit fuel mode. When using either this bug or the preceding one, if you exit fuel mode, you’ll fall straight down, but without taking (or dealing) damage, or using a parachute.
There’s also a way to stay in midair while you shoot, but I haven’t figured out the exact method yet. It seems to involve you sitting on a detached piece of your own shield, holding yourself up by the bootstraps, so to speak. It costs you about half of your shield charge, though, and if you lose shields, you’ll fall, so it might not be worth it.
I’ve only tested these for the little tank on the far right of the selection list, but bug 3 at least should work for all of them.
Also, if you kill another tank without firing a weapon (by bodyslamming or sinkholing), your name at top will change to the name of the tank you killed, but this doesn’t seem to affect gameplay.
Kayby, you figured out a way to duplicate items, and all you used it for was arrows? That shows either some twisted sort of integrity, or a remarkable lack of creativity.
So many bugs, so little time. Unfortunately, 99% of them aren’t exploitable, and so they don’t fall under the definition of ‘cool’.
The first to come to mind is a Return to Castle Wolfenstein bug. If playing as a lieutenant in multiplayer, and you’re looking through your binoculars, you can throw grenades to the spot that you’re looking. That is, zoom in on a spot across the entire map, select a grenade, and throw. Suddenly you’re able to lob a grenade about 500 yards with astounding accuracy.
That’s the elegant way to take out a sniper. Fixed as of the 1.3 patch two days ago.
WOW! That happened to me too! Noone ever believed me. Maybe a power surge or something, but it was like that when I turned the power on.
My favorite glitches:
Double Dragon (NES)-
Floating barrel: In level 1, about halway through the stage is a barrel. If you stand near the door where the whip-wielding Linda’s come out of the building, and chuck it towards the edge, the barrel will magically float all the way up off the screen.
Floating Billy: At the end of the first leg of the stage is a building where you go in and fight Abobo on a conveyor belt. If you walk up against the dead-end wall where that first leg ends, you can make Billy magically fly up the side of the wall and get stuck. The only way back down is to kill yourself by going off the top of the screen. 
The golf club: At the very end of the game, you hafta fight a whole bunch of people, eventually leading up Willy, with his damn gun, and the other Double Dragon. Near the beginning of that battle, Linda will attack you with a whip–if you put her down, and then pick up the whip right before she ‘blinks’ for the last time out of existance, the whip will change by a graphical glitch into something that looks eerily similar, but completely unlike a golf club. You can repeat the trick to carry the weapon over between the waves of baddies–makes it easy to put down Willy.
My favorite glitch of all time, probably because I think I discovered it myself, was from Super Mario Brothers.
You have to have the mushroom, that is, you need to be ‘big’. In one of the castle stages, either run under or jump over Bowser, and stand next to the axe. Once you’re there, he’ll tip-toe over to you, this is where timing is critical.
You need to grab the axe-- -and- get hit by Bowser at the same time! If you do it right, Mario will turn transparent, because he got hit, but Bowser will also die.
Now the game is screwed up–the graphics are working in reverse! You’re ‘big’, but if you get hit, you’ll die. If you grab a mushroom, you’ll turn small. If you get hit while small, you’ll grow big again. It’s very cool.
Not sure if this is a bug or not, but back in the old “Maniac Mansion” PC and NES game, the dungeon contained a security door that could only be unlocked by entering a code number which was the same as the high score on an arcade game in another part of the mansion.
The arcade game is broken for most of the game, however, so the player has to repair some wires to restore it’s power supply. A risky and time consuming process. However, the security door would also accept the high score of the arcade game while it was turned off…Which, being a non-functional game, was “0000”.
It might not be a bug, but I thought it was a nice touch.
Ranchoth
In the early days of the 1.08 patch for Starcraft: Brood War a few months ago, if you built a Creep Colony and damaged it [some specific amount], then morphed it into a Sunken Colony, it was invincible. They fixed that one up with a quickness. It was the most useful of the bugs that came with that patch.
There’s a similar glitch in Final Fantasy IV. Unequip a weapon/shield in combat, then finish the combat unarmed/without the shield. After combat, re-equip it, and you’ll be armed with 2. Unequip, then re-equip. The spare sticks around. Also, I think, works with arrows, but has a strange effect on them - the number doesn’t change, on the display, but it also doesn’t go down when you use them. At least not for a while.
Made it cheaper to keep Blade equipped, as well as giving me lots and lots of gil.
My favorite bug is from Monkey King, a Donkey Kong arcade variation years back. At the level with the barrels, when the jumpman is on the second rung, go to the very edge on the right. Then do a back jump to the right. You don’t die; instead you finish the level.